The present invention relates to a thermosensitive recording material, and more particularly to a thermosensitive recording material capable of forming images with high density and high stability at high speed recording.
In general, a conventional thermosensitive recording material comprises a support material made of, for example, paper or a film, and a thermosensitive coloring layer formed on the support material, which thermosensitive coloring layer comprises a colorless or light colored dyes such as a leuco dye, and a color developer such as a phenolic material (for example, bisphenol A) which is capable of inducing color formation in the colorless or light colored dyes upon application of heat therto, and an acidic material, a binder agent, a filler, a thermosensitivity improvement agent, a lubricating agent and other additives. Examples of such thermosensitive recording material are disclosed, for example, in Japanese Patent Publication No. 43-4160, Japanese Patent Publication No. 45-14039, Japanese Laid-Open Patent Application No. 48-27736 and are widely used in practice.
This thermal recording is attained by a chemical reaction between the colorless or light colored dye and the color developer upon application of heat thereto, for example, by use of a thermal head build-in thermal printer or facsimile apparatus.
As compared with other conventional recording materials, the above described type thermosensitive recording materials have advantages in that recording can be performed by a simple apparatus without requiring complicated steps such as development and image fixing; therefore, such recording is done speedly, quitely, at a low cost and without causing any air pollution problem. Because of these advantages, the thermosensitive recording materials are widely used, for instance, for use with computers, facsimile apparatus, telex, medical measurement instruments and other measuring instruments in a variety of fields.
In accordance with the recent general demand for high speed and highly condensed recording, there is not only a great demand for a high speed thermosensitive recording apparatus, but also a demand for a thermosensitive recording material that is sufficiently compatible with the high speed thermosensitive recording apparatus so as to be capable of attaining high speed and highly condensed recording, in particular, for use in the above-mentioned fields.
Conventionally, thermosensitive recording materials for use in high-speed recording have been proposed, for instance, in Japanese Laid-Open Patent Application No. 53-39139, Japanese Laid-Open Patent Application No. 53-26139, Japanese Laid-Open Patent Application No. 53-5636, and Japanese Laid-Open Patent Application No. 53-11036. In those thermosensitive recording materials, thermo-fusible materials with a low melting point, such as a variety of waxes, fatty acid amides, alkylated biphenyls, substituted biphenyl alkanes, coumarinic acid derivatives, biphenyl amines, are added to the thermosensitive coloring layer as a sensitizer or as an agent for reducing the melting point of the thermosensitive coloring layer. When the above described sensitizer agents are employed, it is necessary that the sensitizer agent be melted prior to the coloring reaction. Therefore, in the high-speed recording by application of a small amount of thermal energy in an extremely short time, for instance, by heat-application impulses, a sufficiently high thermal response for practical use cannot be obtained. In addition to the above problem, in the above case, since the thermo-fusible materials are melted within the thermosensitive coloring layer, the accumulation of the melted thermo-fusible materials on a thermal head during an image recording process, trailing of the printed images and formation of ghost images are apt to occur. Furthermore, at high temperatures and high humidities, fogging also occurs in the background of thermosensitive recording material during storage. As a result, the contrast of the recorded images decreases during storage.
Therefore, it is more preferable not to use the above described sensitizer agents or melting-point reducing agents in the thermosensitive recording materials for use in high speed recording.
Conventional colorless or light-colored dyes which are colored upon application of heat thereto for use in the thermosensitive recording materials, usually melt at high temperatures in the range of 160.degree. C. to 240.degree. C. Therefore, when a thermosensitive coloring layer suitable for high speed recording is prepared without using the above-mentioned sensitizers, it is indispensable to use a color developer with a low melting point and high coloring performance capable of inducing color formation in the colorless or light-colored dyes. The color developers to be used in combination with the colorless or light-colored dyes, in particular, phenolic color developers, are described in many references including Japanese Patent Publication No. 45-14039. Among a large number of phenolic color developers, 2,2-bis(4'-hydroxyphenyl)propane (or Bisphenol A) is preferably used, because of the stability of the quality as the color developer, and the reasonable price and availability. Bisphenol A, however, has the shortcoming that its melting point is as high as 156.degree.-158.degree. C., and its color inducement temperature is high, so that its thermal sensitivity is low.
In contrast to this, in the case where phenolic materials with low melting points, for instance, monohydric phenols, such as 4-t-butylphenol (m.p. 94.degree.-99.degree. C.), .alpha.-naphthol (m.p. 95.degree.-96.degree. C.), and .beta.-naphthol (m.p. 119.degree.-122.degree. C.) are employed, the preservability and stability of the thermal recording materials are so poor that the background thereof is fogged and discolored with time at room temperature during storage. and thermal recording materials containing such monophenols have a characteristic phenolic odor and are not suitable for practical use.
In Japanese Patent Publication No. 54-12819 and Japanese Patent Laid-Open Application No. 55-27253, 2,2-bis(4'-hydroxylphenyl)-n-hexane (m.p. 99.degree.-103.degree. C.) and 1,1-bis(4'-hydroxylphenyl)alkane (the alkane having 3 to 13 carbon atoms) are respectively disclosed as having low melting points and being capable of yielding images in a stable manner when they are employed in thermosensitive recording materials. However, these materials are difficult to synthesize and cannot be obtained easily.
In Japanese Laid-Open Publication No. 56-144193, it is described that a lower alkyl ester and a benzyl ester of p-hydroxybenzoic acid can be easily synthesized, and, by use of the esters, thermosensitive recording materials with high sensitivity can be prepared. However, the thermosensitive recording materials using the esters have the shortcomings that the developed colored portions easily discolor and the phenolic materials are separated in the developed image areas in the form of white crystals or powders.
The above-described conventional thermosensitive recording materials utilizing the chemical coloring reaction between a leuco dye and a phenolic material also has the shortcoming that the developed images formed thereon are discolored or disappear, or the density of the images significantly decreases, while in contact with, for instance, oils and polyvinyl chloride film. In the case of polyvinyl chloride film, this occurs possibly due to the plasticizers contained in the polyvinyl chloride film.
In order to eliminate the above shortcoming, there is disclosed a thermosensitive recording material having an overcoat layer comprising a water-soluble polymeric material for protecting the thermosensitive coloring layer thereof. However, since the overcoat layer hinders heat transmission, when the overcoat layer is formed on the thermosensitive coloring layer, its thermosensitivity decreases. Furthermore, the coating process of the overcoat layer makes the recording material expensive.